1. Technical Field
The present invention relates to an actuator, an optical scanner and an image forming apparatus.
2. Related Art
As an optical device for drawing by optical scanning using a laser printer or the like, various devices have been known in the related art. These devices include an optical device which uses an actuator including a torsional vibrator. (JP-A 2004-191953 is an example of related art.)
JP-A-2004-191953 discloses an actuator including a torsional vibrator of one-degree-of-freedom vibration system. The actuator includes a reflective mirror, a fixed frame section for supporting the reflective mirror, and a pair of spring sections for coupling the reflective mirror to the fixed frame section. The pair of spring sections are provided so as to support the reflective mirror from the both sides of the reflective mirror.
Each of the spring sections has a first spring section for coupling the reflective mirror to the coupled body and a second spring section for coupling the fixed frame section to the coupled body. In addition, the second spring section includes a pair of elastic bodies which are provided so as to be opposed to each other with respect to a central axis of rotation. That is, the spring section is structured such that the spring is branched into two at a middle point thereof.
To each of the second spring sections, there is joined a piezoelectric element (driving source) such that it expands and contracts in the longitudinal direction thereof. The actuator applies voltage to the piezoelectric element and causes the piezoelectric element to expand and contract, causing bending deformation of the second spring section. Along with the deformation, the actuator torsionally deforms the first spring section so as to rotate the reflective mirror, and reflects and scans a light beam. This enables drawing by optical scanning.
In this case, since the piezoelectric element joined to each of the spring sections generates heat caused by driving (voltage application) thereof, the temperature at the spring section is forced to increase. The increase in temperature causes the spring section to expand. However, since the actuator according to JP-A-2004-191953 has a dual-support beam (dual support) structure, it does not enable permitting change in shape of the spring section caused by expansion. This results in a rapid change in a spring constant of the pair of spring sections and distortion of the entire actuator, which disables maintaining stable driving of the actuator.
Particularly in the case where the actuator is used as an optical scanner like the actuator in JP-A-2004-191953, the drawback as described above becomes notable. Specifically when the actuator scans a light beam, the reflective mirror reflects most of the irradiated light beam. However, it is impossible to make optical reflectivity by the reflective mirror 100%. In such an actuator, a part of the light beam which has been irradiated to the reflective mirror turns to heat, increasing the temperature of the actuator. The increased temperature of the actuator causes the drawback as described above.
In addition, thermal expansion of the spring section caused by the increased temperature displaces the central axis of rotation of the reflective mirror. This displacement results in a drawback that the actuator cannot have desired scan characteristics.
Based on what has been described above, the actuator according to JP-A-2004-191953 has a drawback that it cannot maintain desired scan characteristics in the case where it is used for a long period of time.